The invention relates to an article that includes a ceramic and a superalloy and to an article of manufacture that can be fabricated into a part for a gas turbine engine.
Power and efficiency of gas turbine engines typically increase with increasing operating temperature. However, the capability of a turbine engine to operate at increasingly higher temperature is limited by the ability of the turbine components particularly shrouds, vanes and blades, to withstand heat, oxidation and corrosion effects of an impinging hot gas stream.
Improved turbine components have been fabricated from strong stable substrate materials covered with thin protective coatings that resist oxidation and corrosion effects. Such substrates include nickel-based or cobalt based superalloy compositions. Insulative ceramic coatings further enhance turbine performance by reducing heat transferred into components such as airfoils. The coatings can increase durability of components by minimizing the transfer of heat so as to reduce thermal stress.
A ceramic material can be part of a coating system that provides a thermal barrier on an alloy substrate. Typically the coating system has two layers of differing composition and function. A thin (1-2 mil) bond coat, such as NiCrAlY, is first applied to provide an oxide scale and a strong bond to the thicker (10-20 mil) ceramic coating which is subsequently applied to protect the alloy substrate from high temperature oxidation and thermal shock.
A ceramic material can also be used as a material of construction to form turbine components such as combustor liners and shrouds. These components need to be supported mechanically by superalloy structures. The contact zone between the ceramic material supporting superalloy structure in these applications must be capable of withstanding temperatures of 1100xc2x0 C. for long periods of time. Also, tight contact at the interface must be maintained under stress induced by vibration and the interface materials must be chemically compatible.
One ceramic material includes a silicon/silicon carbide (Si/SiC) or a silicon/silicon carbide/carbide (Si/SiC/C) material produced by infiltrating carbon fiber prepregs (woven tows of carbon fibers) with molten silicon. The carbon fibers become mostly but not entirely, converted into silicon carbide (SiC). The final material contains silicon carbide (SiC), free silicon (Si) and carbon.
A direct contact zone between silicon-based ceramic material and superalloy does not meet the above-mentioned requirements. Silicon and carbon diffuse across the silicon-based ceramic:superalloy interface causing degradation of the superalloy physical properties. Other complex reactions precipitate brittle silicide and carbide phases within the superalloy and these phases act as sites for initiation and propagation of cracks. Alumina or yttria diffusion barrier coatings have been proposed to address these problems. However, these coatings fail because of thermal mismatch. For example, sputtered yttria coatings on superalloy or on a silicon-based ceramic, fracture and become detached after only a single thermal treating cycle.
There remains a need for a satisfactory diffusion barrier layer between a substrate formed of a superalloy and a silicon-based ceramic material.
The invention provides a barrier layer that prevents diffusion of silicon and carbon into a superalloy structure. The article of the invention comprises a substrate formed of a superalloy, a silicon-based ceramic material and a thermally stable silicon diffusion barrier layer. The thermally stable diffusion barrier layer lies between superalloy substrate and the ceramic and prevents diffusion of silicon and carbon into the substrate. The diffusion barrier coating is any coating that is thermally stable and that prevents diffusion of silicon or carbon across the ceramic material:superalloy interface.
In another aspect, the invention relates to a method of forming an article, comprising forming a superalloy substrate, applying a thermally stable diffusion barrier layer onto the substrate and applying a silicon-based ceramic material onto the diffusion barrier layer on the substrate, wherein the diffusion barrier layer substantially prevents diffusion of silicon from the ceramic material into the substrate.